Hidetoshi Iwamura

Ultrafast 1.55 μm all‐optical switching using low‐temperature‐grown multiple quantum wells

Low‐temperature grown surface‐reflection all‐optical switching has been demonstrated with ultrafast photoresponse (1.5 ps), low switching energy (2 pJ), high‐contrast (13 dB), polarization independence, and wide operation wavelength range in the 1.55 μm band using low‐temperature‐grown Be‐doped strained InGaAs/InAlAs multiple quantum wells. The combination of low‐temperature growth and Be‐doping contributes to the ultrafast photoresponse. Additionally, the introduction of compressive strain and a mirror with 1% reflectivity greatly enhances optical nonlinearities.

Ultrafast 1.55‐μm photoresponses in low‐temperature‐grown InGaAs/InAlAs quantum wells

Doping with Be was found to be very effective for shortening of carrier lifetime in InGaAs/InAlAs multiple quantum wells (MQWs) grown at low temperature by molecular beam epitaxy. The MQW materials have carrier lifetimes controllable from a few tens of picoseconds to 1 ps in the 1.55‐μm wavelength region, coupled with a large optical nonlinearity due to an excitonic feature, implying applicability to ultrafast optical devices in the fiber‐optic communication. The carrier lifetime was measured by a time‐resolved pump‐probe method using an optical source based on a 1.535‐μm semiconductor laser. We also investigated the resistivity, carrier density, and Hall mobility in the MQWs.

InGaAsP/InAlAs superlattice avalanche photodiode

The fabrication of an InGaAsP-InAlAs superlattice avalanche photodiode using a gas source molecular beam epitaxy is discussed. A quaternal alloy of InGaAsP was used for the well layers in order to suppress the dark current due to the tunneling effect. With this structure, the valance band discontinuity almost vanishes and a gain bandwidth of 110 GHz was obtained. >

Dependence of the GaAs/AlGaAs superlattice ionization rate on Al content

Al content dependence of GaAs/Alx Ga1−xAs superlattice ionization rates was studied. The electron ionization rate is enhanced when the AlGaAs of the barrier is a direct transition type. It is drastically reduced at the Γ‐X band crossover in the AlGaAs layer. The ionization rate ratio (hole to electron) as determined by excess multiplication noise measurement is reduced from a value of 0.5 at x=0.3 to 0.14 at x=0.45. At higher values of x, corresponding to the onset of indirect electron transitions, the noise is increased.

Integrated external-cavity InGaAs/InP lasers using cap-annealing disordering

InGaAs/InP multiple-quantum-well lasers were integrated with low-loss waveguides in the long-wavelength region using a cap-annealing disordering technique which does not require a regrowth process. The coupling efficiency between them does not increase the threshold current of the lasers. The optical loss in the waveguide was 7.8 cm/sup -1/ and was due to free-carrier absorption in the cladding layers.<<ETX>>

Waveguide-Type Optical Modulator of GaAs Quantum Well Double Heterostructures Using Electric Field Effect on Exciton Absorption

This letter describes a GaAs/AlGaAs quantum well (QW) waveguide-type optical modulator utilizing an electric field effect on two-dimensional exciton absorption. The waveguide modulator has the same structure as QW laser diodes, and is therefore suitable for monolithic integration with a laser diode. Optical modulation was measured up to 1 Gbit/s at a driving voltage as low as 1.4 V. The 3 dB-down modulation rate was 900 Mbit/s, but higher speed operation is expected.

Ultrafast high-contrast all-optical switching using spin polarization in low-temperature-grown multiple quantum wells

We have demonstrated an ultrafast high-contrast surface-reflection all-optical switch using a spin-polarization scheme. The device is made with low-temperature-grown Be-doped strained InGaAs/InAlAs multiple quantum wells. It has a photoresponse of 300 fs due to ultrafast electron trapping. The on/off ratios have been drastically improved by more than 20 dB, and reached over 30 and 40 dB at pump pulse energies of 4 and 14 pJ, respectively. Furthermore, we have obtained polarization insensitive operation by handling the two orthogonal components of the signal pulse at two different points.

TE/TM mode selective channel waveguides in GaAs/AlAs superlattice fabricated by SiO2 cap disordering

Novel structure TE/TM mode selective optical channel waveguides are proposed and fabricated in GaAs/AlAs superlattices (SLs) using impurity‐free disordering induced by SiO2 cap annealing. The disordered SL regions are found to be useful as low‐loss waveguides even at a wavelength close to the absorption edge of the as‐grown SL. By observing the near‐field patterns, it is confirmed that these channel waveguides could selectively confine TE or TM polarized light.

Static and dynamic response of multiple-quantum-well voltage-controlled bistable laser diodes

We have simulated the static and dynamic characteristics of voltage-controlled multiple-quantum-well (MQW) bistable laser diodes. To investigate the time response of the saturable absorber under applied electric field, we performed pump-probe measurements with picosecond resolution. The obtained differential transmission signals indicate the reduction of the carrier escape time for the saturable absorber with increasing applied electric field. The field screening effect caused by spatial change of the carrier distribution is an important factor, as is phase space filling due to the photogenerated carriers. On the basis of the time response measurements, we have designed an MQW bistable laser by solving the modified rate equation including the recovery time response of the absorption saturation, A saturable absorption region narrower than 10 /spl mu/m is suitable for obtaining a low threshold device. To achieve low switching power and high switching speed, it is important to optimize the bias conditions and the MQW structures. We can expect a turn-off time of less than 10 ps, and a repetition rate of over 5 GHz from the calculations.

Cross-Sectional Potential Imaging of Compound Semiconductor Heterostructure by Kelvin Probe Force Microscopy

The cross-sectional potential images of the GaAs/AlAs multiple quantum wells and InAlAs/InGaAs heterostructures have been successfully obtained by Kelvin probe force microscopy (KFM). It was shown that the smaller amplitude of the alternating voltage (Vac) applied to detect the electrostatic force during the KFM measurements gave a better potential profile of the heterostructures. The spatial resolution of the present KFM was investigated by measuring the InAlAs/InGaAs layered structures with various thicknesses. The minimum thickness of the InAlAs layer distinguished by the KFM was 40 nm.